Before we speak, we babble...
Before we babble, we listen.
It is the same with babies who are learning to sign. Before they make clear signs they will move their hands around in an attempt to sign. Even before then they will react to other people signing. A smile, a squeak, reaching for the signing hand... these are all reactions that my 5 month old has when she likes what we are signing. Take a look...I sign "milk" to ask if she wants some and in response she gets excited, reaches for my hand and puts my finger in her mouth to suck. When she isn't hungry she'll either push the hand away or turn away.
The other day the 5 month old was on the floor making noises and movements which my husband didn't understand but 3 yr. old big brother knew what she wanted and was able to help by telling daddy that the baby wanted to be picked up.
Signing is a great way to help siblings communicate too!